In certain data processing systems, the main processor may not have the required data processing rate to perform the necessary calculations and to control the I/O devices. In one solution to this problem relating to the data processing capacity, it is known that the load on the main processor can be reduced by providing a secondary processor, controlled by the main processor, which performs the necessary processing for the I/O devices. Such a secondary processor will be herein referred to as a peripheral unit controller.
In prior art systems typically, the main processor transmits high level orders to the peripheral unit controller, and the peripheral unit controller performs the required control on the I/O device to execute the order. Microprocessors or microprogrammed controllers have been used in peripheral unit controllers before as illustrated by U.S. Pat. No. 4,124,889 of Kaufman et al. In Kaufman et al., one peripheral unit controller, which is composed of a microprogrammed controller, alone controls a number of I/O devices. The failure of the microprogrammed controller means that group of I/O devices is unavailable for use by the data processing system. The single microprocessor per peripheral unit controller has proven successful in applications which have low reliability requirements and do not command a great deal of maintainability. Most data processing systems have such requirements. However, systems such as telecommunication systems have greater requirements for reliability and maintainability.